This invention relates to anesthetic gas agent monitors that are used to accurately identify anesthetic agents in a anesthetic gas sample obtained from an anesthetized patient as well as to provide an indication of their relative concentrations.
In the field of medical monitoring equipment it is desired to produce an apparatus for measuring the concentration of anesthetic agents in a respiratory gas sample that is inexpensive, simple to use, accurate and fast in its measurements of anesthetic gas concentrations and the identification of the primary and secondary anesthetic agents. In the field of anesthesiology, there are a number of commonly used anesthetic agents including nitrous oxide, halothane, enflurane, isoflurane, sevoflurane and desflurane. The concentrations of these anesthetic agents are most often measured by an infrared anesthetic agent monitoring apparatus which operates by measuring the optical transmissivity of a respiratory gas sample at certain wavelengths of light. One such anesthetic agent measurement apparatus is disclosed in commonly assigned U.S. Pat. Nos. 5,731,581 and 5,714,759 which disclosures are hereby incorporated by reference. The resultant measurements are processed to identify one of the known anesthetic agents that is contained in the gas sample as well as its concentration.
Infrared anesthetic agent monitoring apparatus generally perform two functions: the identification of the anesthetic agent or agents present in the respiratory gas sample and the determination of the concentration of the identified anesthetic agent or agents. These two functions may be performed either by two separate sets of circuitry in the apparatus or by a common set of circuitry. These apparatus are determined systems, with the number of spectral filters used therein being greater than or equal to the number of anesthetic agents that the apparatus is designed to identify. The selection of the wavelengths passed by the spectral filters is driven by the desire to use wavelengths that are strongly and uniquely absorbed by the anesthetic agents in question. Beer's Law (A=ECL) teaches that the light absorbance exhibited by a respiratory gas sample is substantially linearly related to the concentration of the anesthetic agent contained in the respiratory gas sample. Since the terms E and L in this equation are constants, the presence and partial pressure of the anesthetic agent are determined by monitoring wavelength specific light absorbance values. If each anesthetic agent absorbed energy at only one of the selected wavelengths, the analytical task is simple in that the concentration of each anesthetic agent linearly follows the light absorbance measured at that one wavelength. However, anesthetic agents are chemically similar and their absorbance spectra generally overlap, exhibiting varying degrees of colinearity. This lack of absorbance uniqueness necessitates measuring light absorbance of the respiratory gas sample at all of the selected wavelengths to identify and quantify the anesthetic agent contained in the respiratory gas sample.
After an anesthetic agent is identified its identity is displayed to the user along with concentration data. If the user, i.e., the anesthesiologist, changes the anesthetic agent being given to a patient then the two agents must both be identified with one as the primary agent and the other as the secondary agent. The primary agent should be the anesthetic which is producing the greatest therapeutic effect or drug response in the patient. A problem arises when volume concentration alone is used to determine the primary and secondary agents. A high volume concentration of a less effective anesthetic agent which was originally given by the anesthesiologist can interfere with the proper identification of a more therapeutically effective second agent as the primary agent until the point when the volume concentration of the earlier given first agent decreases below the volume concentration of the second agent.